<directory> is where the individual mbox files are read from/written to.

Type can ONLY be MAIL (SPOOL is NOT supported)

Requires a logger called MailRepository.

Implementation notes:

This class keeps an internal store of the mbox file When the internal mbox
file is updated (added/deleted) then the file will be re-read from disk and
then written back. This is a bit inefficent but means that the file on disk
should be correct.

The mbox store is mainly meant to be used as a one-way street. Storing new
emails is very fast (append to file) whereas reading them (via POP3) is
slower (read from disk and parse). Therefore this implementation is best
suited to people who wish to use the mbox format for taking data out of James
and into something else (IMAP server or mail list displayer)